Method of performing code synchronization, and receiver

ABSTRACT

The invention relates to a method of performing code acquisition on a signal to be received and to a receiver implementing the method. In the method the receiver receives a signal from a transmitter by at least two antenna beams, and to perform code acquisition, the antenna beams formed are searched at different delays of the spreading code. The number M of antenna beams can also be changed adaptively by determining a signal-to-noise ratio and by forming a number of antenna beams which is proportional to the signal-to-noise ratio. The receiver performs code acquisition by receiving a signal from the transmitter and by searching the antenna beams formed at the different delays of the spreading code.

FIELD OF THE INVENTION

[0001] This is a Continuation of International ApplicationPCT/Fl01/00116 filed on Feb. 8, 2001 which designated the U.S. and waspublished under PCT Article 2(2) in English. The invention relates tosynchronization of a receiver with a signal to be received.

BACKGROUND OF THE INVENTION

[0002] In the CMDA (Code Division Multiple Access) the user's narrowbanddata signal is modulated onto a relatively wide band with a spreadingcode which has a wider band than the data signal. In prior art CDMAsystems bandwidths of over 1 MHz are used. In the WCDMA (Wide-band CDMA)the bandwidth is considerably greater so that the users of mobilecommunications networks can be offered a wider range of services.

[0003] The spreading code used in the CDMA system usually consists of along pseudorandom bit sequence. The bit rate of the spreading code ismuch higher than that of the data signal. The spreading code bits arecalled ‘chips’ to distinguish them from the data bits and symbols. Eachof the user's data symbols is multiplied by spreading code chips. Inthat case a narrowband data signal spreads over the frequency band usedby the spreading code. Each user has a spreading code of their own. Thelength of a spreading code may be one or more data bits. Several userstransmit simultaneously on the same frequency band, and the data signalsare distinguished from one another in receivers on the basis of apseudorandom spreading code.

[0004] In spread spectrum systems, such as the CDMA system, the receiverhas to synchronize with the signal to be received so that the signal canbe modulated and detected. Code synchronization is usually divided intotwo parts: code acquisition and code tracking. In code acquisition theobject is to achieve a maximum timing difference of one spreading codechip between the spreading code of the receiver's code generator and thespreading code of the received signal. To achieve this, the spreadingcode of the receiver's code generator is delayed chip by chip, and thedelay between the spreading code of the signal and the spreading code ofthe code generator is searched for. In code tracking the object is toachieve as small timing inaccuracy as possible, which is only a fractionof a chip.

[0005] Code acquisition is performed using e.g. a simple correlator.When the spreading code of the signal to be received and the spreadingcode of the receiver's code generator are not synchronized, the outputof the correlator receives a low value. When the spreading code of thesignal to be received and the spreading code of the receiver's codegenerator are synchronous, i.e. the delay of code generator's spreadingcode is correct, the initial value of the correlator is high. Acorresponding result is obtained when a matched filter is used.

[0006] In such code acquisition problems are caused by interferences andnoise. Multiple access interference MAI, for example, causes highmomentary values at the output of the correlator, even though thespreading code of the signal to be received and the code of the codegenerator would be non-synchronous. This leads to a false alarm andprotracted code acquisition. To reduce the influence of false alarms,the delay obtained in code acquisition has to be confirmed with anothercorrelation measurement, which prolongs synchronization.

BRIEF DESCRIPTION OF THE INVENTION

[0007] An object of the invention is to provide a method and a receiverimplementing the method to reduce the problems related to codeacquisition and to speed up code acquisition. This is achieved with amethod according to claims 1 and 2 and with a receiver according toclaims 13 and 14.

[0008] The preferred embodiments of the invention are disclosed in thedependent claims.

[0009] The invention is based on performing code acquisitiontwo-dimensionally both in the time domain and in the angular domain. Inthe angular domain code acquisition is performed using several adjacentantenna beams, i.e. angular cells.

[0010] The method and arrangement of the invention provide severaladvantages. Code acquisition can be sped up considerably in noisymulti-use environment even when the number of angular cells is notoptimal with respect to the signal to noise ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will be described in greater detail by means ofpreferred embodiments with reference to the accompanying drawings, inwhich

[0012]FIG. 1 illustrates two-dimensional code acquisition,

[0013]FIG. 2 illustrates average duration of code acquisition as afunction of the number of antenna elements at different signal-to-noiseratios,

[0014]FIG. 3 illustrates duration of code acquisition as a function ofthe signal to noise ratio,

[0015]FIG. 4A is a flow chart of code acquisition,

[0016]FIG. 4B is a flow chart of code acquisition,

[0017]FIG. 5 illustrates probability density distribution of thedecision variable that leads to a false alarm, and probability densitydistribution of the decision variable that leads to the correctdecision,

[0018]FIG. 6 illustrates the average duration of code acquisition atdifferent values of the probability of false alarm, and

[0019]FIG. 7 is a block diagram of a receiver.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The solution according to the invention is particularly suitablefor a CDMA radio system which utilizes the direct spreading technique.Other applications include satellite systems, militarytelecommunications systems and private non-cellular networks. However,the solution according to the invention is not limited to theseapplications.

[0021] The present solution will be first described with reference toFIG. 1. A receiver 100, which comprises an antenna array, is a basestation of a radio system, for example. The base station 100 receives asignal from a terminal 102, which is e.g. a mobile station. By means ofthe antenna array the base station 100 can receive signals from adjacentsectors which correspond to the directions of the receiving beams of theantenna array. The number M of antenna beams, which is defined as thenumber of angular cells, is not constant, but the number of angularcells can be changed at different moments. The number M of angular cellsis changed adaptively according to the signal-to-interference ratio(noise is also taken into account in the signal-to-interference ratio).The coverage area, which is independent of the number of receivingbeams, can be divided into two or more sectors. The spread angles of allsectors are preferably equal, even though the spread angles can also beof different sizes in the present solution. Adjacent antenna beamsopening out into the sectors do not overlap. Performance of codeacquisition in different sectors means angular code acquisition in whichthe inaccuracy of code acquisition to be performed in the angular domaindepends on the spread angle of the sector.

[0022] Code acquisition is performed in the time domain in a mannerknown per se, i.e. one goes through all delays of the spreading code.The number of delays to be tested is determined by the number N_(C) ofthe spreading code chips. The number C of delays to be tested during theduration of the spreading code defines a time cell, which may have e.g.a duration of one chip or shorter (the duration of the time cell isselected so that the desired accuracy can be achieved). When the numberof angular cells is M and the number of delays to be searched for is C,code acquisition is performed in two domains in Q=MC cells altogether.Code acquisition can be performed in all cells Q in any order. Forexample, code acquisition can be performed by searching all time cellsin each angular cell, after which the same will be performed in the nextangular cell. Code acquisition can also be performed e.g. by searchingall angular cells in each time cell.

[0023] In the solution according to the invention the essential factorsare the signal-to-noise ratio and the number M of angular cells, i.e.antenna beams. A poor signal-to-noise ratio also increases the codeacquisition time. On the other hand, the signal-to-noise ratio can beimproved with a large number of angular cells because the antenna gainincreases. However, the larger the number of angular cells, the longerthe code acquisition time. FIGS. 2 and 3 show different simulatedsituations which illustrate the code acquisition time as a function ofthe number of angular cells and signal-to-noise ratio. In both figuresthe number C of time cells is 256.

[0024]FIG. 2 illustrates the average code acquisition time T_(ACQ) asmultiples of code sequence, and as a function of the number M of anglecells at different signal-to-noise ratios. The lowest curve 200 shows asituation in which the signal-to-noise ratio SNR is 8 dB, the secondlowest curve 202 a situation in which the SNR=3 dB, and the top curve204 a situation in which the SNR=0 dB. As regards the behaviour of thecurves, it should be noted that the code acquisition time achieves itsminimum with a certain number of angular cells, which depends on thesignal-to-noise ratio. Since in the situation illustrated by curve 200the signal to be received is considerably stronger than theinterference, the code acquisition time is at its shortest with twoangular cells. Nearly the same code acquisition time is also achievedwith one angular cell. When the signal-to-noise ratio decreases to 3 dBin the case of curve 202, the most optimal number of angular cells issix in respect of code acquisition. It should be noted that, compared tothe use of one angular cell, the code acquisition time decreasesconsiderably even with two angular cells. When the signal-to-noise ratiois 0 dB, i.e. the level of the signal to be received is the same as thatof the interference(s), the code acquisition time-is at its shortestwith ten angular cells in the case of curve 204. Furthermore, in thiscase the code acquisition time first decreases rapidly as a function ofthe increased number of angular cells, for which reason it is notnecessary to know the exact signal-to-noise ratio to reduce the codeacquisition time efficiently.

[0025]FIG. 3 illustrates the code acquisition time as a function of thesignal-to-noise ratio with different numbers of angular cells. In thisfigure the code acquisition time is shown on a logarithmic scale. It canbe seen in FIG. 3 that the number M of angular cells needed increasesrapidly as the signal-to-noise ratio deteriorates. When thesignal-to-noise ratio exceeds 9 dB, code acquisition is fastest with oneangular cell. When the signal-to-noise ratio is between 1 dB and 0 dB,it is best to use eight angular cells. In FIGS. 2 and 3 the optimalthreshold is used as the threshold for decision on synchronization.

[0026]FIG. 4A is a flow chart of code acquisition. In block 400 a signalis received using at least two angular cells. In block 402 the delays(time cells) of antenna beams (angular cells) are searched in a desiredorder to find the delay at which the spreading code of the receiver'scode generator synchronizes with the signal spreading code.

[0027]FIG. 4B is also a flow chart of code acquisition. In block 405 anestimate is formed from the ratio of the level of the signal to bereceived to the level of interferences. The levels of the signal and theinterferences are preferably measured as root-mean-square values. Thesignal-to-noise ratio can be estimated only by means of the interferencelevel when it is assumed that the signal level is a constant value orwhen such a value is set for it. In block 452 a number of angular cellswhich is proportional to the signal-to-noise ratio estimate is formed,and a signal is received. If the signal level is assumed to have aconstant value or such a value is set for it, the number of angularcells needed can in practice be determined inversely proportionally withrespect to the signal-to-noise ratio. It is not necessary to determinethe exact number of angular cells by means of the signal-to-noise ratioestimate, but it is sufficient to increase the number of angular cellsby a desired number if the amount of interference increases or theestimated signal-to-noise ratio deteriorates. If the amount ofinterference decreases or the estimated signal-to-noise ratio improves,the number of angular cells can be reduced if more than one angular cellis in use. Code acquisition is performed in block 456 by searching theangular cells formed at the delays of the spreading code.

[0028] It may be difficult to measure the signal-to-noise ratio of thesignal to be received because the signal to be received has not beensynchronized and thus it has not been received for the measurements. Forthis reason, the signal-to-noise ratio of the signal to be receivedoften has to be estimated otherwise.

[0029] When the CFAR principle (Constant False Alarm Rate) is used, theprobability P_(FA) of false alarm can be selected as desired. Theprobability P_(FA) of false alarm means the probability at which codeacquisition results in the wrong delay of the spreading code. Thefollowing procedures are performed when the CFAR principle is used. Theinterference level is measured. Instead of the actual measurement, theinterference level can be estimated e.g. on the basis of the number ofusers. The signal-to-noise ratio can be determined by setting an assumedpower for a synchronization request signal, i.e. it is assumed that thesignal arrives at a certain power. The terminal usually transmits asignal in proportion to the power at which the terminal receives thesignal. Thus the path attenuation is cancelled out and the power of thesignal arriving from the terminal can be estimated at the base station.The signal-to-noise ratio can be used for determining the number ofangular cells needed. However, to be able to calculate the number ofangular cells, one needs information on the threshold T_(H) used fordeciding on synchronization (selection of the optimal threshold maycause problems). This threshold value can be calculated utilizing theassumption that the probability P_(FA) of false alarm is constant. Thusthe probability P_(FA) of false alarm can be selected freely. Thethreshold T_(H) is calculated according to the probability P_(FA) offalse alarm.

[0030] The probability of false alarm will now be described in greaterdetail. The probability function of the decision variable that leads toa false alarm follows the Rayleigh distribution, and the probabilityP_(FA) of false alarm can be calculated by integrating the Rayleighdistribution from the threshold value to infinity as follows${P_{FA} = {{\int_{T_{H}}^{\infty}{\frac{y}{\sigma^{2}}{\exp \left( \frac{- y^{2}}{2\quad \sigma^{2}} \right)}{y}}} = {\exp \left( \frac{- T_{H}^{2}}{2\sigma^{2}} \right)}}},$

[0031] where y is the received signal and y${y = \sqrt{y_{I}^{2} + y_{Q}^{2}}},$

[0032] σ² is the variance of variables Y_(I) and y_(Q), i.e. theroot-mean-square value of interferences, T_(H) is the threshold used fordeciding on synchronization, Y_(I) and y_(Q) are signal componentsaccording to IQ modulation (IQ modulation is obvious to a person skilledin the art). The threshold_(HCFAR) can be calculated from this asfollows T_(HCFAR)={square root}{square root over (−2σ² 1n(P_(FA)))}.

[0033] The probability of finding the correct code delay is calculatedas follows${P_{D} = {\int_{T_{H}}^{\infty}{\frac{y}{\sigma^{2}}{\exp \left( \frac{- \left( {y^{2} + s^{2}} \right)}{2\sigma^{2}} \right)}{I_{0}\left( \frac{ys}{\sigma^{2}} \right)}{y}}}},$

[0034] where s²=m_(I) ²+m_(Q) ² is the parameter of eccentricity, wherem_(I) is the average of signal component y_(I) and m_(Q) is the averageof signal component Y_(Q), I₀ is a modified Bessel function of thezeroth order of the first type. These distributions are shown in FIG. 5.Curve 500 shows the probability density function of aRayleigh-distributed decision variable, and curve 502 shows theRice-distributed probability density function of the correct decisionvariable. The distances between the greatest values of the distributionsdepend on the levels of the interferences and the signal to be received.By setting the threshold value T_(HCFAR) to the desired value, theprobability of correct decision can also be estimated when the level ofthe signal to be received or the signal-to-noise ratio is known, can beestimated or is set.

[0035] The average code acquisition time can be expressed as follows:${{\overset{\_}{T}}_{acq} = {\frac{\left. {2 + {\left( {2 - P_{D}} \right)\left( {Q - 1} \right)}} \right)\left( {1 + {KP}_{FA}} \right)}{2P_{D}}\tau_{d}}},$

[0036] where τ_(d) is the time needed for calculating the decisionvariable (integration time in FIG. 7), K is related to the dead time(Kτ_(d)) of false alarm, and Q=MC. The dead time means the time neededfor recovery from a false alarm. When the number M of angular cells isincreased, the signal-to-noise ratio increases, the probability P_(D) ofcorrect code acquisition increases and the probability P_(FA) of falsealarm decreases. It appears from the formula that this leads to ashorter code acquisition time {overscore (T)}_(acq). The signal-to-noiseratio increases as the number of angular cells is increased, and at thesame time the number Q of cells also increases. The larger the number Qof cells, the longer the code acquisition time {overscore (T)}_(acq)will be. This is explained more closely in Spread Spectrum CommunicationHandbook, M. Simon, J. Omura, R. Scholtz and B. Levitt, McGraw Hill,Inc., 1994, pages 768-770, which is incorporated herein by reference.

[0037]FIG. 6 illustrates the code acquisition time as a function ofangular cells at different P_(FA) values. It is assumed that thesignal-to-noise ratio SNR is 3 dB. The different curves have beenobtained by changing the probability P_(FA) of false alarm, whichchanges the threshold T_(H) for the decision on synchronization. Whenthe threshold for the synchronization decision changes, the codeacquisition time also changes in accordance with the curves. The firstcurve 600 shows a situation in which the probability of false alarm isset to value P_(FA)=0.01. According to this curve, the optimal number Mof angular cells is three or four. The second curve 602 illustrates asituation in which the probability of false alarm is set to valueP_(FA)=0.001. In this case the optimal number M of angular cells is fiveor six. In the case of the third curve 604 the probability of falsealarm is set to value P_(FA)=0.0001. Here the optimal number M ofangular cells is eight. For example, P_(FA)=0.001 could be selected asthe probability of false alarm. The curve 602 corresponding to thisvalue follows rather well an optimal curve 606 defined by means of theactual signal-to-noise ratio. In the solution according to the inventionthe objective is to find a number of angular cells which reduces thecode acquisition time. In that case the best number of angular cells issuch that the code acquisition time reaches its minimum. This applies toFIGS. 2, 3 and 6.

[0038]FIG. 7 is a block diagram illustrating one feasible embodiment ofthe receiver. The receiver comprises blocks that function according tothe prior art, but in respect of code acquisition the function of thereceiver differs essentially from the prior art. Signals are received byan antenna array, which comprises several antenna elements 700 to 704.From the antenna elements 700 to 704 the signals propagate to radiofrequency means 706 to 710, in which the received signal is transferredfrom the radio frequency to the base band. A base band analogue signalis converted into a digital signal in an A/D converter 712 to 716.Digital signals are multiplied by weighting coefficients W_(1,i)-W_(N,i)in multipliers 718 to 722. The values of the weighting coefficientsW_(1,i)-W_(N,i) are determined in a signal-to-noise ratio estimator 750,in which the signal-to-noise ratio is estimated. The signal-to-noiseratio can be estimated in various ways, and thus block 750 can receiveinformation related to the estimation of the signal-to-noise ratio fromseveral parts of the receiver/base station. The weighted signals arecombined in a combiner 724. Multiplication and combination of thesignals provides the desired angular cell. Different coefficients areneeded to determine different angular cells. For this reason, theweighting coefficients form a set (matrix) including N coefficientvectors N, each of which comprises N elements in which N is the maximumnumber of angular cells (corresponds to the number of antenna elements).Each vector at a time is selected as the weighting coefficient forforming a certain antenna beam. The set can be presented as a matrix$\begin{bmatrix}w_{1,1} & \cdots & w_{N,1} \\\vdots & ⋰ & \vdots \\w_{1,N} & \cdots & w_{N,N}\end{bmatrix}\quad$

[0039] where N is the number of antenna elements and the maximum numberof angular cells. The weighting coefficients are typically complexcoefficients. If the number M of angular cells needed is smaller thanthe maximum N number of angular cells, (N-M) weighting coefficients canbe set to zero and M weighting coefficients can be formed, which yieldsM angular cells. Angular cells can also be modified analogically byphasing the radio band signals transmitted to different antenna elementsin a desired way. This fact as well as the use of digital weightingcoefficients for directing and modifying an angular cell are obvious toa person skilled in the art, for which reason these are not described ingreater detail here.

[0040] After the weighted signals have been combined, code acquisitionis performed in the time domain in a manner known per se using anincoherent correlator or a matched filter. In that case the weighted andcombined signal is divided into two parts. The first signal ismultiplied in a multiplier 726 by the signal {square root}{square rootover (2)} cos(ω₀t) of local oscillator (not shown) and the second signalis multiplied by the signal {square root}{square root over (2)} sin(ω₀t)with an orthogonal phase in a multiplier 728. After this, bothmultiplied signals are correlated with a spreading code in correlators730 and 732. Instead of the correlators 730 and 732, a matched filtercan also be used, the filter being matched to the spreading code to besearched for in a manner known per se. The result obtained is squared inmultipliers 734 and 736. Finally, the real results are added in an adder738 and a square root is extracted from the sum in block 740. The resultis connected to a comparator 744 with a switch 742, and the comparatorcompares the result with the threshold value, on the basis of which itis decided whether the code phase is correct.

[0041] Even though the invention has been described with reference to anexample according to the accompanying drawings, it is clear that theinvention is not limited thereto, but may be modified in various wayswithin the scope of the inventive concept defined in the appendedclaims.

What is claimed is:
 1. A method of performing code acquisition on asignal to be received, which is spread-coded with a pseudorandomspreading code, the method comprising forming a signal-to-noise ratioestimate from the ratio of the level of the signal to be received to thelevel of interferences, forming a number of angular cells which isproportional to the signal-to-noise ratio estimate, and receiving asignal; and performing code acquisition by searching the angular cellsformed at different delays of the spreading code.
 2. A method ofperforming code acquisition on a signal to be received, which isspread-coded with a pseudorandom spreading code, the method comprisingreceiving a signal with at least two angular cells, performing codeacquisition by searching the angular cells at the delays of thespreading code.
 3. A method according to claim 1 or 2, forming asignal-to-noise ratio estimate and by determining the number of angularcells needed on the basis of the signal-to-noise ratio estimate.
 4. Amethod according to claim 1 or 2, wherein the lower the level of thesignal to be received in relation to the level of interferences, themore angular cells will be formed.
 5. A method according to claim 1 or2, wherein the angular cells point in different directions.
 6. A methodaccording to claim 1 or 2, wherein the angular cells are adjacent but donot overlap.
 7. A method according to claim 1 or 2, selecting the numberof angular cells so that the code acquisition time is minimized.
 8. Amethod according to claim 1 or 3, estimating, in order to form asignal-to-noise ratio estimate, the level of interferences and making anassumption on the level of the signal to be received.
 9. A methodaccording to claim 1 or 3, assuming, in order to form thesignal-to-noise ratio estimate, that the signal to be received has adesired level and forming the number of angular cells needed inreception, the number being inversely proportional to the level ofinterferences.
 10. A method according to claim 1 or 3, measuring thelevel of interferences, assuming that the signal to be received has acertain level and forming a signal-to-noise ratio.
 11. A methodaccording to claim 10, estimating the level of interferences, settingthe probability of false alarm and forming a threshold by means of whicha decision is made on synchronization.
 12. A method according to claim 1or 3, performing code acquisition in a base station of a radio systemand estimating the level of interferences by means of the number ofusers served by the base station.
 13. A receiver which is arranged toperform code acquisition on a signal to be received, which isspread-coded with a pseudorandom spreading code, the receiver comprisingsignal-to-noise ratio estimation means for determining a signal-to-noiseratio estimate from the ratio of the level of the signal to be receivedto the level of interferences, wherein the receiver being arranged toform a number of angular cells which is proportional to thesignal-to-noise ratio and receive a signal; and perform code acquisitionby searching the angular cells formed at different delays of thespreading code.
 14. A receiver for performing code acquisition on asignal to be received, which is spread-coded with a pseudorandomspreading code, the receiver being arranged to receive a signal by atleast two angular cells, wherein the receiver is arranged to performcode acquisition by searching the angular cells at different delays ofthe spreading code.
 15. A receiver according to claim 14, wherein thereceiver comprises signal-to-noise ratio estimation means fordetermining a signal-to-noise ratio estimate, and the receiver isarranged to determine the number of angular cells needed on the basis ofthe signal-to-noise ratio estimate.
 16. A receiver according to claim 13or 14, wherein the smaller the level of the signal to be received inrelation to the level of interferences, the more angular cells thereceiver is arranged to form.
 17. A receiver according to claim 13 or14, wherein the receiver is arranged to direct the angular cells indifferent directions.
 18. A receiver according to claim 13 or 14,wherein the receiver is arranged to direct the angular cells in parallelwithout overlapping.
 19. A receiver according to claim 13 or 14, whereinthe receiver is arranged to select the number of angular cells so thatthe code acquisition time is minimized.
 20. A receiver according toclaim 13 or 15, wherein to form the signal-to-noise ratio, the receiveris arranged to estimate the level of interferences and make anassumption of the level of the signal to be received.
 21. A receiveraccording to claim 13 or 15, wherein to form the signal-to-noise ratioestimate, the receiver is arranged to set the level of the signal to bereceived to a constant value and form the number of angular cells neededinversely proportionally with respect to the level of interferences. 22.A receiver according to claim 13 or 15, wherein the receiver is arrangedto measure the level of interferences, set an assumed level to thesignal to be received and form the signal-to-noise ratio.
 23. A receiveraccording to claim 13 or 15, wherein the receiver is arranged toestimate the level of interferences, set the probability of false alarmand form a threshold on the basis of which a decision is made onsynchronization.
 24. A receiver according to claim 13 or 14, wherein thereceiver is a base station of a radio system and the level ofinterferences is estimated by means of the number of users served by thebase station.